WO2020238302A1

WO2020238302A1 - Lithium secondary battery electrolyte, preparation method therefor and lithium secondary battery

Info

Publication number: WO2020238302A1
Application number: PCT/CN2020/076844
Authority: WO
Inventors: 马强; 秦德君; 邓耀明; 李阳兴; 徐凡
Original assignee: 华为技术有限公司
Priority date: 2019-05-29
Filing date: 2020-02-26
Publication date: 2020-12-03
Also published as: EP3972029A1; CN112018441B; US20220085413A1; CN112018441A; EP3972029A4

Abstract

Provided in an embodiment of the present invention are a lithium secondary battery electrolyte, comprising lithium salt, an organic solvent and flame retardant agents, wherein the flame retardant agents comprise (pentafluoro)cyclotriphosphazene substituted by an electron-donating group and (pentafluoro)cyclotriphosphazene substituted by an electron-withdrawing group. In the present lithium secondary battery electrolyte, two flame retardant agents of (pentafluoro)cyclotriphosphazene substituted by an electron-donating group and (pentafluoro)cyclotriphosphazene substituted by an electron-withdrawing group are added at the same time, such that the battery has both high safety performance and good electrochemical performance. Also provided in embodiments of the present invention are a preparation method for the lithium secondary battery electrolyte and a lithium secondary battery containing the lithium secondary battery electrolyte.

Description

Lithium secondary battery electrolyte and preparation method thereof and lithium secondary battery

Technical field

The embodiments of the present invention relate to the technical field of lithium secondary batteries, in particular to lithium secondary battery electrolytes and preparation methods thereof, and lithium secondary batteries.

Background technique

With the continuous expansion of the use of electronic consumer products such as mobile phones, digital cameras, and notebook computers, higher and higher requirements have been placed on the energy density of lithium-ion batteries, and the increase in battery energy density will also bring about battery safety issues. At present, the electrolyte of lithium-ion batteries is mainly non-aqueous organic electrolyte (normally carbonate electrolyte). When the battery is abused (thermal shock, overcharge, acupuncture and external short circuit, etc.), the electrolyte is easy to exist. Potential hazards such as volatilization and flammability can easily cause safety problems caused by battery thermal runaway.

In view of the flammability problems of the above carbonate electrolytes, the current mainstream solution is to add flame retardant additives to conventional electrolytes, although there are many reported flame retardant additives, such as phosphorus compounds, halogenated compounds, and nitrogen-containing compounds. Compounds, silicon compounds, etc., but most of them have poor compatibility with positive and negative materials. Even if the phosphazene flame retardant with good compatibility with the positive and negative materials is used, in order to ensure the safety of high energy density batteries, it still needs to be added at a relatively high amount (10-20wt.%) to achieve good flame retardancy. effect. However, the addition of a large amount of single phosphazene flame retardant (>15wt.%) will cause electrolyte layering or lithium salt precipitation, which will affect the electrochemical performance of the battery. In order to solve this problem, the industry uses phosphazene flame retardants and other non-phosphazene flame retardants to add in combination to reduce the use of phosphazene flame retardants, but non-phosphazene flame retardants not only have poor flame retardant effects, but also Moreover, the compatibility with positive and negative materials is poor, which affects the safety performance and electrochemical performance of the battery.

Summary of the invention

In view of this, an embodiment of the present invention provides an electrolyte for a lithium secondary battery by adding a (pentafluoro) ring triphosphazene substituted with an electron donating group and a (pentafluoro) ring substituted with an electron withdrawing group to the electrolyte at the same time The two flame retardants of triphosphazene enable the battery to have both high safety performance and good electrochemical performance, so as to solve to a certain extent that the addition of a large number of phosphazene flame retardants will cause electrolyte delamination or lithium salt precipitation. Eventually lead to the problem of poor electrochemical performance of the battery.

Specifically, the first aspect of the embodiments of the present invention provides an electrolyte for a lithium secondary battery, which includes a lithium salt, an organic solvent, and a flame retardant, and the flame retardant includes (pentafluoro)cyclotriphosphorus substituted with electron-donating groups. Nitrile and electron withdrawing group substituted (pentafluoro) cyclotriphosphazene.

In the embodiment of the present invention, the electron-donating group-substituted (pentafluoro) cyclotriphosphazene includes one of alkoxy (pentafluoro) cyclotriphosphazene and phenoxy (pentafluoro) cyclotriphosphazene Or multiple.

In the embodiment of the present invention, in the alkoxy (pentafluoro) cyclotriphosphazene, the number of carbon atoms of the alkoxy group is 1-20.

In the embodiment of the present invention, the alkoxy (pentafluoro) cyclotriphosphazene includes one or more of methoxy (pentafluoro) cyclotriphosphazene and ethoxy (pentafluoro) cyclotriphosphazene .

In the embodiment of the present invention, the (pentafluoro) cyclotriphosphazene substituted by the electron withdrawing group includes fluoroalkoxy (pentafluoro) cyclotriphosphazene and alkylsulfonic acid group (pentafluoro) cyclotriphosphazene One or more of.

In the embodiment of the present invention, in the fluoroalkoxy (pentafluoro) cyclotriphosphazene, the number of carbon atoms of the fluoroalkoxy is 1-20.

In the embodiment of the present invention, the fluoroalkoxy (pentafluoro) cyclotriphosphazene includes trifluoroethoxy (pentafluoro) cyclotriphosphazene and perfluorobutoxy (pentafluoro) cyclotriphosphazene. One or more of.

In the embodiment of the present invention, in the alkylsulfonic acid group (pentafluoro) cyclotriphosphazene, the alkylsulfonic acid group has 1-20 carbon atoms.

In the embodiment of the present invention, the alkylsulfonic acid (pentafluoro) cyclotriphosphazene includes methylsulfonic acid (pentafluoro) cyclotriphosphazene and ethylsulfonic acid (pentafluoro) cyclotriphosphazene. One or more of.

In the embodiment of the present invention, the mass percentage of the (pentafluoro)cyclotriphosphazene substituted by the electron donating group in the electrolyte is 10%-30%.

In the embodiment of the present invention, the mass percentage of the (pentafluoro)cyclotriphosphazene substituted with the electron withdrawing group in the electrolyte is 1% to 5%.

In the embodiment of the present invention, the total mass percentage of the (pentafluoro) cyclotriphosphazene substituted by the electron donating group and the (pentafluoro) cyclotriphosphazene substituted by the electron withdrawing group in the electrolyte The content is 16%-30%.

In the embodiment of the present invention, the electrolyte further includes other additives, and the other additives include biphenyl, fluorobenzene, vinylene carbonate, trifluoromethyl ethylene carbonate, vinyl ethylene carbonate, 1,3-propane Sultone, 1,4-butane sultone, vinyl sulfate, vinyl sulfite, succinonitrile, adiponitrile, 1,2-bis(2-cyanoethoxy)ethane and 1, One or more of 3,6-hexane trinitrile.

In the embodiment of the present invention, the mass percentage of the other additives in the electrolyte is 0.1%-20%.

In the embodiment of the present invention, the lithium salt includes LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAsF ₆ , LiPF ₂ O ₂ , LiCF ₃ SO ₃ , LiTDI, LiB(C ₂ O ₄ ) ₂ (LiBOB), LiBF ₂ C ₂ O ₄ (LiDFOB), Li[(CF ₃ SO ₂ ) ₂ N], Li[(FSO ₂ ) ₂ N] and Li[(C _m F _2m+1 SO ₂ )(C _n F _2n+1 SO ₂ ) One or more of N], where m and n are natural numbers.

In the embodiment of the present invention, the molar concentration of the lithium salt in the electrolyte is 0.01 mol/L-2.0 mol/L.

In the embodiment of the present invention, the organic solvent includes a carbonate-based solvent and/or a carboxylate-based solvent.

In the embodiment of the present invention, the carbonate-based solvent is a mixed solvent composed of cyclic carbonate and linear carbonate.

In the embodiment of the present invention, the cyclic carbonate includes ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), gamma-butyrolactone (GBL), butylene carbonate (BC) ); the chain carbonate includes one of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and dipropyl carbonate (DPC) Kind or more.

In the embodiment of the present invention, the mass percentage of the cyclic carbonate in the electrolyte is 5%-70%, and the mass percentage of the chain carbonate in the electrolyte is 5% -70%.

In the embodiment of the present invention, the carboxylic acid ester solvent includes methyl acetate (MA), ethyl acetate (EA), propyl acetate, butyl acetate, propyl propionate (PP), and butyl propionate. One or more; the mass percentage of the carboxylic acid ester solvent in the electrolyte is 5%-30%.

In the lithium secondary battery electrolyte provided by the first aspect of the embodiments of the present invention, the (pentafluoro) cyclotriphosphazene substituted by the electron donating group and the (pentafluoro) cyclotriphosphazene substituted with the electron withdrawing group are simultaneously added to the electrolyte. The two flame retardants of phosphazene, under the synergistic effect of the two flame retardants, further improve the solubility of the phosphazene flame retardant in the electrolyte, and better overcome the problems of electrolyte delamination or lithium salt precipitation. Ensure the electrochemical performance and safety performance of the battery.

In the second aspect, an embodiment of the present invention also provides a method for preparing an electrolyte for a lithium secondary battery, including the following steps:

In a glove box filled with argon, dissolve the fully dried lithium salt in an organic solvent, stir and mix to form a uniform solution, and then replace the electron-donating group (pentafluoro) cyclotriphosphazene with the electron-withdrawing group (Pentafluoro)cyclotriphosphazene is added to the uniform solution, and after mixing uniformly, an electrolyte for a lithium secondary battery is obtained.

The preparation method provided in the second aspect of the embodiment of the present invention has a simple process and is suitable for industrial production.

In a third aspect, an embodiment of the present invention also provides a lithium secondary battery, including a positive electrode, a negative electrode, a separator, and an electrolyte. The electrolyte adopts the lithium secondary battery electrolyte described in the first aspect of the present invention.

Description of the drawings

Fig. 1 is a photograph of the electrolyte of a lithium secondary battery prepared in Example 1 of the present invention;

2 is a photograph of the electrolyte of the lithium secondary battery prepared in Comparative Example 1 of the present invention;

Figure 3 is a photo of the electrolyte of the lithium secondary battery prepared in Comparative Example 2 of the present invention;

4 is a graph of the cycle performance at room temperature of lithium secondary batteries of Example 1-2 and Comparative Example 1-2 of the present invention.

Detailed ways

The embodiments of the present invention will be described below in conjunction with the drawings in the embodiments of the present invention.

In order to improve the safety performance of lithium-ion batteries, the industry has added flame retardants to conventional electrolytes. Among them, the phosphazene flame retardant has good compatibility with the positive and negative materials, and the flame retardant performance is better. However, in order to ensure the safety of high energy density batteries, a relatively high addition amount (10-20wt.%) is required to achieve a good flame retardant effect. However, the addition of a large amount of single phosphazene flame retardant will cause electrolyte layering or lithium salt precipitation, which will affect the electrochemical performance of the battery. To solve this problem, the industry uses phosphazene flame retardants and other non-phosphazene flame retardants to add in combination to reduce the use of phosphazene flame retardants, but non-phosphazene flame retardants not only have poor flame retardant effects, Moreover, the compatibility with the positive and negative materials is poor, which affects the safety performance and electrochemical performance of the battery. In view of this, an embodiment of the present invention provides an electrolyte for a lithium secondary battery, in which a (pentafluoro) ring triphosphazene substituted with an electron donating group and a (pentafluoro) ring substituted with an electron withdrawing group are added to the electrolyte at the same time The two flame retardants of triphosphazene can make the battery have both high safety performance and good electrochemical performance.

Specifically, an embodiment of the present invention provides an electrolyte for a lithium secondary battery, including a lithium salt, an organic solvent, and a flame retardant, wherein the flame retardant includes (pentafluoro) cyclotriphosphazene substituted with electron-donating groups and absorption Electron group substituted (pentafluoro) cyclotriphosphazene.

In the lithium secondary battery electrolyte of the embodiment of the present invention, by adding two types of phosphazene flame retardants with similar structures to the conventional electrolyte at the same time, the synergistic effect of the two types of phosphazene flame retardants has the following beneficial effects: 1) Due to the different polarity and solubility of the two types of flame retardants, the existence of (pentafluoro) cyclotriphosphazene substituted with electron withdrawing groups can break the (pentafluoro) cyclotriphosphazene substituted with electron donating groups in the electrolyte The saturated solubility limit of phosphazene, increase the total usage of phosphazene flame retardant, avoid electrolyte delamination or lithium salt precipitation, improve the flame resistance of electrolyte, and effectively guarantee the safety of the battery; 2) Two different polar phosphorus The mixed use of nitrile flame retardants can well control the amount of (pentafluoro) cyclotriphosphazene substituted with electron withdrawing groups, which can prevent a large number of electron withdrawing groups substituted (pentafluoro) cyclotriphosphazene from reducing in the negative electrode. Film formation causes the battery impedance to be too large; it can also improve the high-voltage capability of the electrolyte, effectively inhibit the oxidative decomposition caused by the electrolyte contact with the positive electrode material under high voltage, and improve the high-voltage cycle performance of the battery; 3) Two types of flame retardants The structure is similar, which can effectively avoid the problem of poor compatibility between different flame retardants and ensure that the battery has good electrochemical performance.

In the embodiment of the present invention, the structural formulas of (pentafluoro) cyclotriphosphazene substituted with electron donating groups and (pentafluoro) cyclotriphosphazene substituted with electron withdrawing groups can be expressed by the structural formula shown in formula (I):

When R is an electron donating group, it is a (pentafluoro) cyclotriphosphazene substituted with an electron donating group; when R is an electron withdrawing group, it is a (pentafluoro) cyclotriphosphazene substituted with an electron withdrawing group Nitrile.

In some embodiments of the present invention, the electron-donating group may be an alkoxy group or a phenoxy group, that is, the (pentafluoro) cyclotriphosphazene substituted by the electron-donating group includes an alkoxy (pentafluoro) cyclotriphosphazene and One or more of phenoxy (pentafluoro) cyclotriphosphazene. In some other embodiments, the electron-donating group may also be another electron-donating group that can form a bond with the phosphorus atom of cyclotriphosphazene. In some embodiments of the present invention, the number of carbon atoms of the alkoxy group can be 1-20, further, the number of carbon atoms of the alkoxy group can be 1-10, and further, the number of carbon atoms of the alkoxy group can be 2-6. The alkoxy group may be linear or branched. In some specific embodiments, the alkoxy (pentafluoro) cyclotriphosphazene may include one or more of methoxy (pentafluoro) cyclotriphosphazene and ethoxy (pentafluoro) cyclotriphosphazene .

In some embodiments of the present invention, the electron withdrawing group may be a fluoroalkoxy group or an alkylsulfonic acid group, that is, the electron withdrawing group substituted (pentafluoro) cyclotriphosphazene includes fluoroalkoxy (pentafluoro ) One or more of cyclotriphosphazene and alkylsulfonic acid (pentafluoro) cyclotriphosphazene. In some other embodiments, the electron withdrawing group may also be another electron withdrawing group that can form a bond with the phosphorus atom of cyclotriphosphazene.

In some embodiments of the present invention, the number of carbon atoms of the fluoroalkoxy group is 1-20. Further, the number of carbon atoms of the fluoroalkoxy group can be 1-10. Furthermore, the number of carbon atoms of the fluoroalkoxy group is 1-10. The number of carbon atoms can be 2-6. In the embodiment of the present invention, the fluoroalkoxy group may be a perfluoroalkoxy group or a partially fluoroalkoxy group. The fluoroalkoxy group may be straight or branched. In some specific embodiments, the fluoroalkoxy (pentafluoro) cyclotriphosphazene includes trifluoroethoxy (pentafluoro) cyclotriphosphazene and perfluorobutoxy (pentafluoro) cyclotriphosphazene One or more.

In some embodiments of the present invention, the number of carbon atoms of the alkyl sulfonate group is 1-20. Further, the number of carbon atoms of the alkyl sulfonate group can be 1-10. The number of carbon atoms can be 2-6. The alkylsulfonic acid group may be linear or branched. In some specific embodiments, the alkylsulfonic acid group (pentafluoro) cyclotriphosphazene includes methylsulfonic acid group (pentafluoro) cyclotriphosphazene and ethylsulfonic acid group (pentafluoro) cyclotriphosphazene One or more.

In the embodiment of the present invention, the mass percentage of the (pentafluoro)cyclotriphosphazene substituted with the electron-donating group in the electrolyte may be 10%-30%. Further, the mass percentage of (pentafluoro)cyclotriphosphazene substituted with electron-donating groups in the electrolyte may be 15%-25%.

In the embodiment of the present invention, the mass percentage of (pentafluoro)cyclotriphosphazene substituted with electron withdrawing groups in the electrolyte is 1% to 5%. The addition of (pentafluoro) cyclotriphosphazene substituted by electron withdrawing group can effectively increase the total usage of phosphazene flame retardant and improve the flame retardancy of electrolyte. And controlling it to a relatively small amount can not only prevent a large number of electron-withdrawing groups substituted (pentafluoro) cyclotriphosphazene from being reduced to a film on the negative electrode, resulting in a large battery impedance; but also can increase the high voltage of the electrolyte Ability to effectively inhibit the oxidation and decomposition of the electrolyte caused by contact with the positive electrode material under high voltage, and improve the high voltage cycle performance of the battery.

In the embodiment of the present invention, the total mass percentage of the electron-donating group-substituted (pentafluoro)cyclotriphosphazene and the electron-withdrawing group-substituted (pentafluoro)cyclotriphosphazene in the electrolyte is 11% -35%, further may be 16%-30%, and still further, may be 20%-25%.

In the embodiment of the present invention, other additives may be included in the electrolyte according to actual needs. Specifically, the other additives may include one or more of film-forming additives, high-voltage additives, anti-overcharge additives, and interface wetting agents , Wherein the film-forming additives can be but not limited to vinylene carbonate, trifluoromethyl ethylene carbonate, vinyl ethylene carbonate, 1,3-propane sultone, 1,4-butane sultone, sulfuric acid Vinyl ester, vinyl sulfite, high voltage additives can be but not limited to succinonitrile, adiponitrile, 1,2-bis(2-cyanoethoxy)ethane and 1,3,6-hexane trinitrile One or more of. The anti-overcharge additive may be biphenyl, for example, and the interfacial wetting agent may be fluorobenzene, for example. The mass percentage content of other additives in the electrolyte may be 0.1%-20%, and further may be 0.5-10%.

In the embodiment of the present invention, the lithium salt may be a commonly used conductive lithium salt, and specifically may include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAsF ₆ , LiPF ₂ O ₂ , LiCF ₃ SO ₃ , LiTDI, LiB(C ₂ O ₄ ) ₂ (LiBOB), LiBF ₂ C ₂ O ₄ (LiDFOB), Li[(CF ₃ SO ₂ ) ₂ N], Li[(FSO ₂ ) ₂ N] and Li[(C _m F _2m+1 SO ₂ ) One or more of (C _n F _2n+1 SO ₂ )N], where m and n are natural numbers. In some embodiments of the present invention, the molar concentration of the lithium salt in the electrolyte may be 0.01 mol/L-2.0 mol/L.

In the embodiment of the present invention, in order to coordinate the dissolution of the flame retardant with a high added amount, the organic solvent can be a carbonate-based solvent and/or a carboxylate-based solvent. Among them, the carbonate-based solvent may be a mixed solvent composed of a cyclic carbonate and a chain carbonate. Specifically, the cyclic carbonate may include one of ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), gamma-butyrolactone (GBL), and butylene carbonate (BC). One or more; the chain carbonate may include one or more of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and dipropyl carbonate (DPC).

In some embodiments of the present invention, the mass percentage of the cyclic carbonate in the electrolyte may be 5%-70%, and the mass percentage of the chain carbonate in the electrolyte may be 5%-70%.

In the embodiment of the present invention, the carboxylic acid ester solvent may include one of methyl acetate (MA), ethyl acetate (EA), propyl acetate, butyl acetate, propyl propionate (PP), and butyl propionate. One or more; the mass percentage of the carboxylic acid ester solvent in the electrolyte can be 5%-30%.

In the lithium secondary battery electrolyte provided by the embodiments of the present invention, by adding two types of phosphazene flame retardants with different polarities and similar structures to the electrolyte at the same time, the total usage of the phosphazene flame retardants is increased, and the battery is While having high safety performance, it effectively guarantees the electrochemical performance of the battery, and the two types of phosphazene flame retardants have good compatibility with the positive and negative materials, without deteriorating other aspects of the battery performance (such as low temperature and rate performance), and has a broader Application prospects.

Correspondingly, the embodiment of the present invention also provides a preparation method of the above-mentioned lithium secondary battery electrolyte, which includes the following steps:

In a glove box filled with argon, dissolve the fully dried lithium salt in an organic solvent, stir and mix to form a uniform solution, and then replace the electron-donating group (pentafluoro) cyclotriphosphazene with the electron-withdrawing group (Pentafluoro)cyclotriphosphazene is added to the above-mentioned homogeneous solution, and after mixing homogeneously, an electrolyte for lithium secondary battery is obtained.

The preparation method provided by the embodiment of the present invention has a simple process and is suitable for industrial production.

An embodiment of the present invention also provides a lithium secondary battery, including a positive electrode, a negative electrode, a separator, and an electrolyte, and the electrolyte adopts the lithium secondary battery electrolyte of the foregoing embodiment of the present invention. The lithium secondary battery provided by the embodiment of the present invention has a relatively large amount of phosphazene flame retardant added to its electrolyte, and the phosphazene flame retardant has good compatibility with the positive and negative electrode materials and has good flame retardancy, so that the battery It has both excellent safety performance and electrochemical performance.

The following further describes the embodiments of the present invention in multiple embodiments.

Example 1

An electrolyte for lithium secondary batteries, including lithium salts (LiPF ₆ and LiDFOB), a non-aqueous organic solvent formed by mixing EC, DEC, PC, FEC and PP in a weight ratio of 25:20:25:5:15, and Including the flame retardants ethoxy (pentafluoro) cyclotriphosphazene and trifluoroethoxy (pentafluoro) cyclotriphosphazene, the concentration of LiPF ₆ is 1.0 mol/L, and the concentration of LiDFOB is 0.05 mol/L The mass percentages of ethoxy (pentafluoro) cyclotriphosphazene and trifluoroethoxy (pentafluoro) cyclotriphosphazene are 15% and 2%, respectively.

Preparation of the electrolyte for the lithium secondary battery in this embodiment:

In a glove box filled with argon, EC, DEC, PC, FEC and PP are mixed to form an organic solvent, then fully dried LiPF ₆ and LiDFOB are dissolved in the above solvent, stirred and mixed into a uniform solution, and then the ethoxy group The (pentafluoro) cyclotriphosphazene and the trifluoroethoxy (pentafluoro) cyclotriphosphazene were added to the above homogeneous solution, and then mixed uniformly to prepare the lithium secondary battery electrolyte of Example 1 of the present invention.

Production of lithium secondary battery:

Weigh 2% polyvinylidene fluoride (PVDF), 2% conductive agent super P, and 96% lithium cobalt oxide (LiCoO ₂ ), and add them to N-methylpyrrolidone (NMP) in turn, stir and mix thoroughly Obtain the slurry uniformly, coat the obtained slurry on the aluminum foil current collector, dry, cold press, and cut to obtain the positive electrode sheet;

Weigh the mass percentages as 1.5% CMC, 2.5% SBR, 1% acetylene black and 95% graphite, add them to deionized water in turn, stir and mix well to obtain slurry, and coat the obtained slurry on the copper foil current collector On, drying, cold pressing, and slitting to obtain the negative pole piece;

The positive pole piece, the negative pole piece and the commercial PP/PE/PP three-layer separator prepared above were made into batteries, and polymer packaging was used to infuse the lithium secondary battery electrolyte prepared in Example 1 of the present invention. After the process, a 3.8 Ah soft-packed lithium secondary battery was produced.

Example 2

An electrolyte for lithium secondary batteries, including lithium salts (LiPF ₆ and LiDFOB), a non-aqueous organic solvent formed by mixing EC, DEC, PC, FEC and PP in a weight ratio of 25:20:25:5:15, and Including flame retardants ethoxy (pentafluoro) cyclotriphosphazene and methylsulfonic acid group (pentafluoro) cyclotriphosphazene, the concentration of LiPF ₆ is 1.0 mol/L, and the concentration of LiDFOB is 0.05 mol/L The mass percentages of ethoxy (pentafluoro) cyclotriphosphazene and methylsulfonic acid (pentafluoro) cyclotriphosphazene are 15% and 2%, respectively.

In a glove box filled with argon, EC, DEC, PC, FEC and PP are mixed to form an organic solvent, then fully dried LiPF ₆ and LiDFOB are dissolved in the above solvent, stirred and mixed into a uniform solution, and then ethoxylated The (pentafluoro)cyclotriphosphazene and the methylsulfonic acid group (pentafluoro)cyclotriphosphazene were added to the above homogeneous solution, and then mixed uniformly to prepare the lithium secondary battery electrolyte of Example 2 of the present invention.

The production of the lithium secondary battery was the same as in Example 1.

Example 3

An electrolyte for lithium secondary batteries, including lithium salts (LiPF ₆ and LiDFOB), a non-aqueous organic solvent formed by mixing EC, DEC, PC, FEC and PP in a weight ratio of 25:20:25:5:15, and Including flame retardants ethoxy (pentafluoro) cyclotriphosphazene, trifluoroethoxy (pentafluoro) cyclotriphosphazene and methylsulfonic acid group (pentafluoro) cyclotriphosphazene, among which the concentration of LiPF ₆ 1.0mol/L, the concentration of LiDFOB is 0.05mol/L, ethoxy (pentafluoro) cyclotriphosphazene, trifluoroethoxy (pentafluoro) cyclotriphosphazene and methanesulfonate (pentafluoro) The mass percentages of cyclotriphosphazene are 15%, 1% and 1% respectively.

In a glove box filled with argon, EC, DEC, PC, FEC and PP are mixed to form an organic solvent, then fully dried LiPF ₆ and LiDFOB are dissolved in the above solvent, stirred and mixed into a uniform solution, and then ethoxylated (Pentafluoro) cyclotriphosphazene, trifluoroethoxy (pentafluoro) cyclotriphosphazene and methylsulfonic acid group (pentafluoro) cyclotriphosphazene are added to the above solution and mixed uniformly to prepare the embodiment 3 of the present invention Lithium secondary battery electrolyte.

The production of the lithium secondary battery was the same as in Example 1.

Comparative example 1

In a glove box filled with argon, EC, DEC, PC, FEC, and PP are mixed to form an organic solvent, and then fully dried LiPF ₆ and LiDFOB are dissolved in the above solvent, and stirred and mixed to uniformly prepare the comparative example 1 of the present invention. Of electrolyte. The concentration of LiPF ₆ is 1.0 mol/L, the concentration of LiDFOB is 0.05 mol/L, and the mass percentages of EC, DEC, PC, FEC and PP are 25:20:25:5:15, respectively. The production of the lithium secondary battery was the same as in Example 1.

Comparative example 2

In a glove box filled with argon, EC, DEC, PC, FEC and PP are mixed to form an organic solvent, then fully dried LiPF ₆ and LiDFOB are dissolved in the above solvent, stirred and mixed into a uniform solution, and then ethoxylated (Pentafluoro)cyclotriphosphazene was added to the above solution and mixed thoroughly to prepare the electrolyte of Comparative Example 2 of the present invention. Among them, the concentration of LiPF ₆ is 1.0mol/L, the concentration of LiDFOB is 0.05mol/L, the mass percentages of EC, DEC, PC, FEC and PP are 25:20:25:5:15, and the ethoxy (pentafluoro ) The mass percentage of cyclotriphosphazene is 17%. The production of the lithium secondary battery was the same as in Example 1.

The electrolytes and lithium secondary batteries in Examples 1-3 and Comparative Examples 1-2 of the present invention were subjected to the following tests:

(1) Test of electrolyte viscosity and conductivity

Use a rotary viscometer to test the viscosity of the electrolyte. Take a 0.5mL-1mL electrolyte sample and place it in the sample table of the viscometer. The test condition is 25℃, the measurement range of the rotor is 1-100mPa/s, and the measurement speed is 50rpm. The viscosity of the electrolyte sample was tested 3 times and the average value was taken. Use a conductivity meter to test the conductivity of the electrolyte. Take 1mL-5mL electrolyte samples and place them in the test tube of the conductivity meter. The test temperature is 25℃. The conductivity of each electrolyte sample is tested 3 times and the average is taken. value.

(2) Test of self-extinguishing performance of electrolyte

Take 1.0g of electrolyte and place it in a 5.0mL crucible, ignite and test its self-extinguishing time. Use the ignition device to quickly ignite, and record the time from when the ignition device is removed to the automatic extinguishment of the flame, which is the self-extinguishing time (SET). The SET test of each electrolyte sample was performed 5 times and the average value was taken. Take the self-extinguishing time per unit mass of electrolyte as the standard to compare the flame retardant properties of different electrolytes.

(3) Performance test of lithium secondary battery

The battery was subjected to a charge-discharge cycle test at a charge-discharge rate of 0.7/0.7C. The voltage range of the graphite/LiCoO ₂ battery was 3.0-4.5V, and the capacity retention rate was recorded for 200 weeks.

The results of the above tests of Examples 1-3 and Comparative Examples 1-2 are listed in Table 1, Figure 1, Figure 2, Figure 3, and Figure 4.

Table 1 Test results of Examples 1-3 and Comparative Examples 1-2

It can be seen from Table 1 that compared with Comparative Example 1, the electrolytes in Examples 1-3 and Comparative Example 2 have excellent flame resistance, which is mainly due to the electrolysis in Examples 1-3 and Comparative Example 2. The liquid contains a phosphazene flame retardant which exhibits high-efficiency flame-retardant properties. When the electrolyte is heated, the phosphazene group in it will decompose to produce P-based free radicals. Capture the H or OH radicals generated by the thermal decomposition of the electrolyte and cut off Chain reaction, thereby improving the flame resistance of the electrolyte; at the same time, we can also see that although the introduction of phosphazene flame retardant in Examples 1-3 will increase the viscosity of the electrolyte and decrease the conductivity of the electrolyte, it is not Comparison of Comparative Example 2 shows that the combination of two types of phosphazene flame retardants has a smaller effect on the viscosity and conductivity of the electrolyte. This is due to the fact that both types of flame retardants are phosphazene compounds with similar structures and can be effective Avoid the problem of poor compatibility between different additives, and the synergistic effect of (pentafluoro) cyclotriphosphazene substituted with electron donating group and (pentafluoro) cyclotriphosphazene substituted with electron withdrawing group can further improve the flame retardancy of phosphazene The total amount of the agent used will not cause delamination or precipitation of lithium salts in the electrolyte.

In addition, Figure 1, Figure 2, and Figure 3 are photos of the electrolyte of Example 1, Comparative Document 1 and Comparative Example 2, respectively. It can be seen from Table 1 and Figures 1, 2 and 3 that in Examples 1-3 The prepared electrolyte is in a clear, transparent and uniform state, while the electrolyte prepared in Comparative Example 2 is turbid and a little lithium salt is precipitated at the bottom. This is mainly due to the amount of ethoxy (pentafluoro) cyclotriphosphazene used in the electrolysis The liquid reaches the upper limit. In Example 1 of the present invention, ethoxy (pentafluoro) cyclotriphosphazene and trifluoroethoxy (pentafluoro) cyclotriphosphazene are simultaneously introduced, effectively avoiding a single ethoxy (pentafluoro) cyclotriphosphazene. (Fluorine) the limit on the amount of cyclotriphosphazene used. At the same time, we can see from Figure 4 that compared to Comparative Example 1-2, the battery in Example 1-2 has better high-voltage cycling performance, and the battery exhibits a higher capacity retention rate after 200 cycles of cycling. This is mainly due to the introduction of (pentafluoro) cyclotriphosphazene substituted with an appropriate amount of electron withdrawing groups, which not only improves the high voltage capability of the electrolyte, but also the electron withdrawing groups (such as trifluoroethoxy or methyl) Sulfonic acid group) can form a dense interfacial film on the surface of the positive electrode material of the battery under high voltage conditions, effectively inhibiting the oxidative decomposition caused by direct contact between the electrolyte and the positive electrode material under high voltage, and avoiding serious side reactions. Improve the high voltage cycle performance of the battery. Therefore, the lithium secondary battery electrolyte provided by the embodiments of the present invention not only solves the limitation of the use of a single phosphazene flame retardant and avoids the problem of poor compatibility with the electrolyte, but also effectively ensures the electrochemical performance of the battery. Performance and safety performance.

Claims

An electrolyte for a lithium secondary battery, which is characterized by comprising a lithium salt, an organic solvent and a flame retardant, the flame retardant comprising (pentafluoro) cyclotriphosphazene substituted with electron donating groups and substituted with electron withdrawing groups的(pentafluoro)cyclotriphosphazene.
The lithium secondary battery electrolyte according to claim 1, wherein the (pentafluoro) cyclotriphosphazene substituted by the electron donating group includes an alkoxy (pentafluoro) cyclotriphosphazene and a phenoxy group. One or more of (pentafluoro)cyclotriphosphazene.
3. The lithium secondary battery electrolyte according to claim 2, wherein in the alkoxy (pentafluoro) cyclotriphosphazene, the number of carbon atoms of the alkoxy group is 1-20.
The lithium secondary battery electrolyte according to claim 3, wherein the alkoxy (pentafluoro) cyclotriphosphazene includes methoxy (pentafluoro) cyclotriphosphazene and ethoxy (pentafluoro) cyclotriphosphazene. ) One or more of cyclotriphosphazene.
The lithium secondary battery electrolyte according to claim 1, wherein the (pentafluoro) cyclotriphosphazene substituted by the electron withdrawing group comprises a fluoroalkoxy (pentafluoro) cyclotriphosphazene and an alkane One or more of sulfonic acid group (pentafluoro) cyclotriphosphazene.
The lithium secondary battery electrolyte according to claim 5, wherein in the fluoroalkoxy (pentafluoro) cyclotriphosphazene, the number of carbon atoms of the fluoroalkoxy is 1-20 .
The lithium secondary battery electrolyte according to claim 6, wherein the fluoroalkoxy (pentafluoro) cyclotriphosphazene comprises trifluoroethoxy (pentafluoro) cyclotriphosphazene and perfluoro One or more of butoxy (pentafluoro) cyclotriphosphazene.
The lithium secondary battery electrolyte according to claim 5, wherein in the alkylsulfonic acid group (pentafluoro) cyclotriphosphazene, the alkylsulfonic acid group has 1-20 carbon atoms .
The lithium secondary battery electrolyte according to claim 8, wherein the alkylsulfonic acid group (pentafluoro) cyclotriphosphazene includes methylsulfonic acid group (pentafluoro) cyclotriphosphazene and ethyl One or more of sulfonic acid (pentafluoro) cyclotriphosphazene.
The lithium secondary battery electrolyte according to claim 1, wherein the weight percentage of (pentafluoro)cyclotriphosphazene substituted by the electron donating group in the electrolyte is 10%-30 %.
The lithium secondary battery electrolyte according to claim 1, wherein the weight percentage of (pentafluoro)cyclotriphosphazene substituted by the electron withdrawing group in the electrolyte is 1%-5 %.
The lithium secondary battery electrolyte according to claim 1, wherein the (pentafluoro) cyclotriphosphazene substituted with the electron donating group and the (pentafluoro) cyclotriphosphazene substituted with the electron withdrawing group The total mass percentage of nitrile in the electrolyte is 16%-30%.
The lithium secondary battery electrolyte according to claim 1, wherein the electrolyte further includes other additives, and the other additives include biphenyl, fluorobenzene, vinylene carbonate, trifluoromethyl ethylene carbonate , Vinyl ethylene carbonate, 1,3-propane sultone, 1,4-butane sultone, vinyl sulfate, vinyl sulfite, succinonitrile, adiponitrile, 1,2-bis( One or more of 2-cyanoethoxy)ethane and 1,3,6-hexanetrinitrile.
11. The lithium secondary battery electrolyte according to claim 13, wherein the mass percentage of the other additives in the electrolyte is 0.1%-20%.
The lithium secondary battery electrolyte according to claim 1, wherein the lithium salt comprises LiClO 4 , LiBF 4 , LiPF 6 , LiAsF 6 , LiPF 2 O 2 , LiCF 3 SO 3 , LiTDI, LiB(C 2 O 4 ) 2 (LiBOB), LiBF 2 C 2 O 4 (LiDFOB), Li[(CF 3 SO 2 ) 2 N], Li[(FSO 2 ) 2 N] and Li[(C m F 2m+1 One or more of SO 2 )(C n F 2n+1 SO 2 )N], where m and n are natural numbers.
The lithium secondary battery electrolyte according to claim 1, wherein the molar concentration of the lithium salt in the electrolyte is 0.01 mol/L-2.0 mol/L.
The lithium secondary battery electrolyte according to claim 1, wherein the organic solvent includes a carbonate-based solvent and/or a carboxylic acid ester-based solvent.
17. The lithium secondary battery electrolyte according to claim 17, wherein the carbonate-based solvent is a mixed solvent composed of a cyclic carbonate and a chain carbonate.
The lithium secondary battery electrolyte according to claim 18, wherein the cyclic carbonate comprises ethylene carbonate, propylene carbonate, fluoroethylene carbonate, γ-butyrolactone, butylene carbonate One or more; the chain carbonate includes one or more of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and dipropyl carbonate.
The lithium secondary battery electrolyte according to claim 18, wherein the mass percentage of the cyclic carbonate in the electrolyte is 5%-70%, and the chain carbonate is in the electrolyte. The mass percentage content in the electrolyte is 5%-70%.
The lithium secondary battery electrolyte according to claim 17, wherein the carboxylic acid ester solvent comprises methyl acetate, ethyl acetate, propyl acetate, butyl acetate, propyl propionate, butyl propionate One or more of esters; the mass percentage of the carboxylic acid ester solvent in the electrolyte is 5%-30%.
A preparation method of lithium secondary battery electrolyte, which is characterized in that it comprises the following steps:

In a glove box filled with argon, dissolve the fully dried lithium salt in an organic solvent, stir and mix into a uniform solution, and then replace the electron-donating group (pentafluoro) cyclotriphosphazene with the electron-withdrawing group (Pentafluoro)cyclotriphosphazene is added to the uniform solution, and after mixing uniformly, the lithium secondary battery electrolyte is obtained.
A lithium secondary battery, which is characterized by comprising a positive electrode, a negative electrode, a separator, and an electrolyte, and the electrolyte adopts the lithium secondary battery electrolyte according to any one of claims 1-21.